15

Phosphorus management inorganic soils for sustainableagricultureJose González1,2, Mark Healy2, Karen Daly1

1Teagasc, Johnstown Castle, Wexford.

2Civil Engineering, National University of Ireland, Galway

jose.gonzalez@teagasc.ie

Introduction

There is an increasing pressure onorganic soils (peats, bogs andassociated soils) as a consequence ofthe growing demand to bring moreland into pasture and hence increasethe milk and meat production.However, these soils have beenidentified as vulnerable to phosphorus(P) loss due to their poor P retentioncapacities when fertilizer P is applied:significant differences in the P sorptionmechanisms in soils take placedepending on the content of organicmatter as there are competitivesorption reactions matter for the soilsorption sites between P and theorganic acids (humic and fulvic acids,low molecular weight acids) derivedfrom the decomposition of this organic.Hence, soil organic matter contentplays an important role regarding Pdynamics with regard to agriculturaland environmental management.These findings have implications forsustainable use of fertilizer P on peatysoils. Nutrient application to these soilsrequires a different managementstrategy compared to mineral soils dueto the high potential for P transfer towater. The aim of this research work isto optimize phosphorus use on soilswith a high content of organic matterthrough the execution of a series ofagronomic and environmentalexperiments in lab conditions that willhelp us to describe P assimilation andtransport in these soils.

Material and Methods

A growth chamber experiment (Figure1) was conducted on six different soilsranging in organic matter content todetermine the agronomic optimumfertilizer P application for ryegrassproduction. The soils were placed inpots (30 cm diameter x 30 cm depth)and 14 different P rates ranging from 0to 145 kg P ha-1 were applied to eachsoil type. Soils were kept at controlledconditions of 14° C (± 2° C) day timeand 8° C (± 2° C) dark period, 70 %relative humidity at day period and 90% relative humidity dark period, a daylength cycle of 16 hours and 8 hdarkness.

Figure 1. Growth chamber trial withthe pots containing the differentsoils under controlled conditions oftemperature and relative humidity.

Dry matter yield and herbage contentwere measure during a period of ninemonths. The original Mitscherlich

16

equation ( ) was used tofit the yield response to the different Ptreatments.

From an environmental point of view, aleaching experiment is currently beingconducted to ascertain the amount ofP lost from two contrasting soils (oneorganic, one mineral). Soils werepacked in 0.3m-deep and 0.104 m-diameter PVC columns. P applicationsof 15, 30 and 55 kg P ha-1 as a singlesuperphosphate fertilizer were appliedin either one or two application times.One hundred and eighty ml of distilledwater is applied weekly on the surfaceof the columns to recreate real rainfallconditions in Ireland. The leachingwater is collected in individualcontainers and analysed weekly forDissolved Reactive P (DRP), Total P(TP) and Total Dissolved P (TDP),along with other nutrients (totalnitrogen and carbon)

Results and Discussion

Cumulative yield response curves tothe different fertiliser P applicationrates from the growth chamberexperiment are shown in Figure 2.Soils were grouped in organic (A) andmineral (B) based on the percentageof organic matter (OM) content(organic soils have OM > 20 %,minerals have OM < 20 %).

Under P deficient scenarios, organicsoils (Figure 2 A) showed a quickerresponse to the P applications thanmineral soils. Mineral soils exhibitedslower response, possibly due to build-up required before P is made plantavailable. This can be due to the abilityof mineral soils to absorb and bindphosphate ions into the clay mineralswhen they are deficient in P. Incontrast, the quick response onorganic soils suggests that the Papplied was not bound but immediatelyavailable for grass uptake. The modelfor the organic soils explained a 56.2% of the total variation whereas is was

lower for mineral soils, only 24.5 %,due to build-up requirement of thesesoils.

Figure 2. Response of thecumulative yield to fertilizer P forcombined organic soils (A) andmineral soils (B)

Conclusions

Fertiliser P requirements for pastureproduction are higher for organic thanfor mineral soils. However, furtherwork on leaching and runoffexperiments is required in order tounderstand if these requirements posea risk of P loss to the environment.

Acknowledgements

This work is funded by the Departmentof Agriculture, Food and the MarineResearch Stimulus Fund 2007-2013.

B

A